EIFAC/ICES WGEEL Report 2008 561
Table SE.i Amounts (kg) of ascending young eels caught in eight rivers along the Swedish coasts.
MOTALA RÖNNE GÖTA RIVER DALÄLVEN STRÖM MÖRRUMSÅN KÄVLINGEÅN Å LAGAN VISKAN ÄLV
RBD YEAR/RBD RBD 2 RBD 4 RBD 4 RBD 4 RBD 5 RBD 5 RBD 5 5 1900 530,0 1901 5100,0 1902 340,0 1903 858,0 1904 552,0 1905 8700,0 1906 2000,0 1907 275,0 1908 na 1909 na 1910 na 1911 5728,0 1912 6529,0 1913 20,0 1914 2828,0 1915 na 1916 na 1917 45,0 na 1918 4,5 na 1919 na 1465,0 1920 na 800,0 1921 na 1555,0 1922 na 455,0 1923 na 1732,0 1924 na 4551,0 1925 na 331,3 5463,0 1926 49,0 357,8 3893,0 1927 445,0 581,1 4796,0 1928 0,0 211,9 47,0 1929 0,0 4,5 756,0 1930 147,0 268,0 5753,0 1931 na 316,0 2103,0 1932 na 408,0 7238,0 1933 na 303,5 6333,0 1934 na 236,0 6338,0 1935 na 53,5 1336,0 1936 na 24,5 2537,0 1937 na 0,5 8711,0 1938 na 106,5 3879,0 1939 na 36,0 4775,0
562 EIFAC/ICES WGEEL Report 2008
MOTALA RÖNNE GÖTA RIVER DALÄLVEN STRÖM MÖRRUMSÅN KÄVLINGEÅN Å LAGAN VISKAN ÄLV 1940 na 684,0 1894,0 1941 na 321,0 2846,0 1942 14,0 na 454,0 427,0 1943 283,0 na 1248,0 1848,0 1944 773,0 na 1090,0 2342,0 1945 406,0 na 1143,0 2636,0 1946 280,0 29,7 766,5 2452,0 1947 272,5 5,8 440,8 675,0 1948 120,0 6,0 494,7 1702,0 1949 43,0 39,4 603,6 1711,0 1950 304,5 93,5 419,9 2947,0 1951 210,0 2713,0 1,0 281,8 1744,0 1952 324,0 1543,5 9,1 379,1 3662,0 1953 241,5 2698,0 70,0 802,4 5071,0 1954 508,5 1030,0 2,7 511,3 1031,0 1955 550,0 1871,0 42,6 506,9 2732,0 1956 215,0 429,0 14,1 501,6 1622,0 1957 161,5 826,0 46,8 336,1 1915,0 1958 336,7 172,0 73,2 497,2 1675,0 1959 612,6 1837,0 80,0 910,5 1745,0 1960 289,0 799,0 29,0 93,0 552,4 1605,0 1961 303,0 706,0 665,5 143,7 314,8 269,0 1962 289,0 870,0 534,8 113,0 261,9 873,0 1963 445,4 581,0 241,2 32,5 298,1 1469,0 1964 158,0 181,6 177,8 34,7 27,5 622,0 1965 276,4 500,0 292,3 87,1 28,0 746,0 1966 157,5 1423,0 196,3 48,5 216,5 1232,0 1967 331,8 283,0 353,6 6,6 24,4 493,0 1968 265,5 184,0 334,8 398,0 74,4 849,0 1969 333,7 135,0 276,8 85,7 117,1 1595,0 1970 149,8 2,0 80,4 29,8 24,7 1046,0 1971 242,0 1,0 141,1 53,3 45,3 12,0 842,0 1972 87,6 51,0 139,9 249,0 106,2 88,0 810,0 1973 159,7 46,0 375,0 282,3 107,1 177,0 1179,0 1974 49,5 58,5 65,4 120,7 33,6 13,0 631,0 1975 148,7 224,0 93,3 206,7 78,4 99,0 1230,0 1976 44,0 24,0 147,2 17,1 20,2 501,0 798,0 1977 176,4 353,0 89,6 32,1 26,4 850,0 256,0 1978 35,1 266,0 168,4 10,8 75,8 532,6 873,0 1979 34,3 112,0 61,4 56,1 165,9 505,2 190,0 1980 71,2 7,0 36,5 165,7 226,0 72,5 906,0 1981 6,8 31,0 72,8 49,2 78,0 513,1 40,0 1982 0,5 22,0 129,0 40,0 90,8 472,0 882,0 1983 112,1 12,0 204,6 37,6 87,8 308,4 113,0
EIFAC/ICES WGEEL Report 2008 563
MOTALA RÖNNE GÖTA RIVER DALÄLVEN STRÖM MÖRRUMSÅN KÄVLINGEÅN Å LAGAN VISKAN ÄLV 1984 33,9 48,0 189,9 0,5 68,0 20,7 325,0 1985 69,7 15,2 138,1 0,0 234,1 211,5 77,0 1986 28,4 26,0 220,3 8,6 2,5 150,9 143,0 1987 73,5 201,0 54,5 84,8 69,8 140,9 168,0 1988 69,0 169,5 241,0 4,9 191,7 91,9 475,0 1989 na 35,2 30,0 0,0 44,0 32,7 598,0 1990 na 21,0 72,5 32,0 21,6 42,1 149,0 1991 na 2,0 151,0 na na 161,3 0,4 264,0 1992 9,6 108,0 14,0 12,5 na 42,2 70,3 404,0 1993 6,6 89,0 45,7 25,8 na 8,7 43,4 64,0 1994 71,9 650,0 283,0 4,0 na 30,7 76,1 377,0 1995 7,6 32,0 72,4 2,9 na 11,6 5,5 0,0 1996 17,5 14,0 51,9 13,5 na 2,8 10,0 277,0 1997 7,5 8,1 148,0 19,4 10,4 31,7 7,6 180,0 1998 14,7 5,5 12,9 15,3 24,0 62,6 5,0 0,0 1999 15,5 85,0 84,2 22,2 4,2 49,5 1,8 0,0 2000 12,4 270,1 1,0 5,0 na 13,0 14,1 0,0 2001 8,2 177,5 19,3 34,5 1,8 26,8 1,8 0,0 2002 58,6 338,8 37,4 19,3 27,0 102,0 26,2 693,0 2003 126,1 19,0 11,0 9,7 9,1 31,7 45,1 266,0 2004 26,4 42,0 1,5 248,3 2,0 29,0 5,0 125,0 2005 30,9 24,8 2,5 3,4 0,1 20,5 25,8 105,0 2006 35,1 25,9 2,5 94,4 0,1 38,1 2,7 0,04 2007 19 >30 112,6 76 4,45 77 2,1 0 2008 >30,5 na na na na >25 >3,4 >0
The ascent in River Viskan is totally dominated by elvers that arrived as glass eels the same year. Also in River Lagan there is a considerable proportion of “glass eels” but in the remaining rivers there is a mix of year classes, with eels up to more than 300 mm in TL. No data available = na. 0 for River Göta Älv in recent years (except in 2007) is as a consequence of the fact the eel pass was closed in those years. Data for 2008 are only indicated as the season is not over yet.
564 EIFAC/ICES WGEEL Report 2008
Ascending young eels in eight rivers
9000
8000 Göta Älv Dalälven 7000 Motala Ström Viskan Lagan 6000 Mörrumsån Rönneå 5000 Kävlingeån
4000 Catch (kg) Catch
3000
2000
1000
0 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 Year
Ascending young eels in eight rivers, from 1950 onw ards
1000 5000
900 4500
Dalälven 800 4000 Viskan
700 Lagan 3500 Mör r ums ån 600 Rönneå 3000 Kävlingeån 500 2500 Göta Älv
Catch (kg) Catch Motala Str öm 400 2000
300 1500 RiversGöta Älv andMotala Ström 200 1000
100 500
0 0 1950 1960 1970 1980 1990 2000 Year
Figure SE.10 a and b Long‐term trends in the catches of young eels at various places along the Swedish coast. The lower panel is a magnified version of the upper one from 1950 onwards.
EIFAC/ICES WGEEL Report 2008 565
Recruitment indices for young eels
1500 2500
1400
1300 Göta Älv Dalälven 1200 Viskan 2000 Lagan 1100 Mörrumsån Rönneå 1000 Aritmetic mean from all seven rivers Motala Ström 900 1500 Geometric mean for all seven rivers 800
700
600 1000 % of average for 1971-80 for average % of 500
400 ström) (Motala 1971-80 for % average of
300 500
200
100
0 0 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 Year
Figure SE.11 Recruitment indices from seven Swedish rivers. Data are presented as percentages of the averages for 1971 to 1980 in the same rivers, respectively.
SE.G.1.2 Recruitment surveys/marine data
The abundance of glass eels in the open sea (Kattegat and Skagerrak) is surveyed by trawling with either an Isaacs‐Kidd Midwater trawl (IKMT) or with a modified Methot‐Isaacs‐Kidd Midwater trawl (MIKT). The former trawl is used in a fixed posi‐ tion in the intake canal for cooling water to the condensers at the Ringhals Nuclear Power Station (e.g. Westerberg, 1998a; 1998b). The latter method is used from RV Ar‐ gos during the ICES‐International Young Fish Survey (since 1993 called the Interna‐ tional Bottom trawl Survey (IBTS Quarter 1) (Hagström and Wickström, 1990).
When the glass eels have settled they and larger eels can be monitored on soft and shallow bottoms using a “Drop Trap” technique (Westerberg et al., 1993). This was successfully done during a number of years but is now a resting series. This approach made it possible to roughly estimate the total recruitment of young eels to the Swed‐ ish coast.
From all three methods recruitment series could be compiled:
Recruitment of glass eel to the Swedish west coast is monitored at the intake of cool‐ ing water to the nuclear power plant at Ringhals in the Kattegat (Figure SE.12 and Table SE.j). The time of arrival of the glass eels to the sampling site varies between years, probably as a consequence of hydrographical conditions, but the peak in abundance normally occurred in late March to early April. Abundance has decreased by 90% if recent years are compared to the peak in the early 1980s. Applying a transi‐ tion function to the data suggests a break in the trend in the early 1980s (Figure SE.13).
566 EIFAC/ICES WGEEL Report 2008
Glass eel abundance, Ringhals 1981-2008
900
800 y = -14,977x + 446,53 700 R2 = 0,3167 600
500
n/night 400
300
200
100
0 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 2007
Figure SE.12 Time trend in glass eel recruitment at the Ringhals nuclear power plant on the Kat‐ tegat coast in Swedish RBD 5 (Västerhavet).
Ringhals Rank 1 Eqn 8078 LorCum(a,b,c,d) r^2=0.19673849 DF Adj r^2=0.18691266 FitStdErr=257.13193 Fstat=26.778421 a=126.51382 b=522.29325 c=1983.4434 d=-0.3056258 2500 2500
2000 2000
1500 1500
No. eels 1000 1000 No. eels
500 500
0 0 1980 1990 2000 2010 year
Figure SE.13 A transition function fitted to the glass eel data from Ringhals.
EIFAC/ICES WGEEL Report 2008 567
Table SE.j Annual indices of glass eel recruitment at the intake canal for cooling water to reactors 1 and 2 at the Ringhals nuclear power plant. Mean of weekly means of numbers of glass eels col‐ lected with a modified Isaacs‐Kidd midwater trawl during March and April (weeks 9–18). Data were corrected for variations in water flow.
week 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 33 1 40 171 4 0 5 4 8 151418305400107422762010 6 2827135645711011014208651122428 7 6 22 9 85 331 7 41 0 22 9 8 267 3 154 2 2 62 3 4 27 8 1 34 57 3 44 57 8 48 11 3 50 12 115 5 327 5 0 22 2 12 17 9 187 51 3 36 342 185 3 160 55 3 172 0 68 125 62 344 5 117 5 1 15 6 11 10 10 199 24 2 80 372 150 15 471 118 7 224 4 200 100 121 377 3 200 10 3 10 2 29 31 11 250 130 528 176 4 19 129 150 88 290 130 610 333 13 198 8 72 533 22 366 44 3 39 1 81 114 12 374 806 835 289 14 6 2 16 107 145 42 469 535 400 569 25 60 177 158 214 24 530 53 18 162 13 382 38 13 1886 1258 265 122 109 1 0 72 291 251 110 562 495 1430 331 60 42 220 2 479 16 59 185 35 153 17 186 30 14 2093 1335 469 181 0 3 31 149 121 351 138 151 403 1236 625 33 77 448 314 942 22 185 192 65 162 55 101 43 15 1849 878 112 878 141 603 67 284 414 298 540 1145 91 128 201 237 377 154 45 184 151 55 202 97 191 26 16 925 476 69 416 42 120 254 142 527 619 64 73 49 96 79 299 25 53 74 90 286 132 20 13 17 804 477 171 350 6 127 37 193 231 564 278 80 56 44 202 141 257 128 8 158 32 66 62 18 2 18 0 297 114 124 55 230 31 9 46 8 10 36 7 mean 9-18 849 711 553 175 305 45 52 169 184 186 138 283 374 636 277 44 117 164 147 400 32 171 92 31 110 42 102 34
The numbers of glass eels caught during the Swedish parts of the International Bot‐ tom trawl Survey (IBTS Quarter 1) are given in Figure SE.14.
Number of glass eels per haul and m2
0,014
0,012
0,01
0,008
0,006 Numbers
0,004
0,002
0 1991 1993 1995 1997 1999 2001 2003 2005 2007 Year
Figure SE.14 Catch of glass eels by a modified Methot‐Isaacs‐Kidd Midwater trawl (MIKT) in the Skagerrak‐Kattegat 1991–2008. Numbers have been corrected for the flow through the net. There were no glass eels caught in 2008.
568 EIFAC/ICES WGEEL Report 2008
SE.G.1.3
Another way of estimating the occurrence of young eels ascending in smaller streams is by electro‐fishing (Degerman, 1985; Fiskeriverket & Laxforskningsinstitutet, 1999; CEN 2002). Normally this is done with salmonids in focus with eels as secondary product or spin‐off.
80
60
40 Eel occurrence (%) occurrence Eel
20
0
199219911990 1993 199619951994 19981997 200120001999 2002 20072006200520042003
Figure SE.15 Proportion of electro‐fished stations (%) with eel occurrence (+/‐95% CI) along the West Coast (only the county of Halland). The stations that were fished in 1990–2007 are situated from 0 to 100 m asl. Note that local abundance is not given here, only presence/absence. Data from SERS (Swedish Electrofishing Register). The trend is not significant (Pearson correlation, n=18, r=0,36, p=0,144).
EIFAC/ICES WGEEL Report 2008 569
80
60
40 Eel occurrence (%) occurrence Eel
20
0
199219911990 1993 199619951994 19981997 200120001999 2002 20072006200520042003
Figure SE.16 Proportion of electro‐fished stations (%) with eel occurrence (+/‐95% CI) along the East Coast. Stations that were fished in 1990–2007 in this figure are situated from 0 to 100 m asl in six counties along the Baltic Sea Coast. Note that local abundance is not given here, only pres‐ ence/absence. Data from SERS (Swedish Electrofishing Register). The negative trend is significant (Pearson correlation, n=18, r=‐0,68, p=0,002)
SE.G.2 Yellow eel surveys
SE.G.2.1 Yellow eel surveys in coastal waters
The coastal fish communities on the Swedish west coast are monitored by standard‐ ized fishing with fykenets in shallow water (2–5 m). Yellow eel was among the domi‐ nating fish species in August most years. Barsebäck in the SW part of the area belongs to RBD SE Baltic, other areas to RBD Västerhavet. The trend for the longest time‐ series from Vendelsö in N Kattegatt is significantly positive. A negative tendency for the Barsebäck area was broken by increasing catches in 2006 and 2007. In the other areas the period of sampling was too short to be examined for biologically significant trends. The magnitude of cpue though, was similar to that of the longer series. The interannual variations in cpue were influenced by water temperature at the time of sampling, but no time‐trends in temperature were observed for the period with avail‐ able data (1988–2007).
570 EIFAC/ICES WGEEL Report 2008
3 25
2,5 20
2 15 1,5 10
n/fykenet day * 1
5 0,5
0 0 1976 1981 1986 1991 1996 2001 2006
Barsebäck Kullen Vendelsö Hakefjorden Lysekil Fjällbacka Water temperature Linjär (Vendelsö)
Figure SE.17 Time trend in the yellow eel catches in coastal fish monitoring with fykenets in Au‐ gust on the Swedish west coast. RBD SE Baltic (Barsebäck) and RBD Västerhavet (others). Annual mean water temperature at the fishing gears is presented for the Vendelsö area in central Kat‐ tegat.
SE.G.2.2 Yellow eel surveys in fresh water
There are no routine stock surveys for yellow eels in fresh water. The nearest equiva‐ lents are the surveys dedicated to stocked populations of eels. These are mostly per‐ formed in smaller lakes but also at one site in the large Lake Mälaren where glass eels were stocked in both 1980 and 1997. The aim is to follow the development of the in‐ troduced stock and individual growth of young eels stocked in nature. The eels that were stocked in 1997 were marked with Alizarin Complexone. Such marked eels are now dominating the local eel population. Their proportion of the catch has increased from 4% in 2000 to 69% in 2007. In 2007 the stocked eels were 494 mm (+/‐75 SD) which corresponds to a growth rate of 39,8 mm/year (+/‐7,5 SD) after stocking. An‐ other 96 eels from the sampling in 2008 are still waiting to be processed.
SE.G.3 Silver eel surveys
There are no regular silver eel surveys in Sweden. However, in 2003 the Institute of Freshwater Research collected large samples from the commercial fisheries in eight lakes and at two sites where most silver eels try to leave the Baltic Sea, i.e. in the Sound (Öresund). In 2005 and 2006 silver eels from additional sites along the Baltic Coast were collected for a tagging study. All these eels (except tagged but not recap‐ tured individuals) have now been analysed with respect to e.g. their fat content and to their chemical background (by otolith microchemistry). This extensive study might together with a now realized tag‐recapture study be the baseline for recurrent sam‐ pling of silver eels. A complementary tag‐recapture study is planned for 2008, where silver eels from both Lake Mälaren and the Stockholm Archipelago will be tagged. Useful data from individual eels will by that be collected.
The Coastal Institute is sampling the commercial catch with the purpose to collect length and age data. This is done within the DCR (Data Collection Regulation Pro‐ gramme). See also Section SE.H below.
EIFAC/ICES WGEEL Report 2008 571
SE.H Catch composition by age and length
SE.H.1 Catch composition by age and length in coastal areas
In 2002–2007 over 8800 yellow eel were sampled for individual length, total and so‐ matic weight, sex and prevalence of Anguillicola crassus. All but 80 were female and the males were mainly recorded on the Skagerrak coast in SD 20. Age readings exist for 2700 individuals, but the major part of the otolits were stored and not analysed after the year of catch 2005 (Table SE.k(b)). The sampling programme started as an initiative of the Swedish Board of Fisheries and is now part of the Swedish contribu‐ tion to the DCR. Sampling of silver eel in poundnet catches started in 2005. So far length and weight recordings and otoliths were collected from 2500 silver eels and 1200 age readings were performed.
Table SE.k Swedish sampling of yellow eel in commercial catches with fykenets.
a. total number sampled for size and age Year of catch ICES SD 2002 2003 2004 2005 2006 2007* Total 20 202 201 200 729 670 723 2725 21 205 198 200 202 100 104 1009 23 202 201 200 200 197 200 1200 25 409 405 414 1 23 1252 27 392 426 469 465 478 392 2622 Total 1410 1431 1483 1596 1446 1442 8808 *in database 20080814 b. total number of age records Year of catch ICES SD 2002 2003 2004 2005 2006 2007 Total 20 97 96 98 433 724 21 98 99 98 201 100 596 23 96 96 198 199 589 25 97 99 1 197 27 390 188 578 Total 291 388 883 1021 101 0 2684
Sampling for length in commercial fykenet catches demonstrate a similar size compo‐ sition of yellow eel in the Kattegat, the Öresund area and on the southern Baltic coast (SD 21, 23 and 25). Sizes in the interval 40–50 cm were most abundant. In Subdivision 20 on the Skagerrak coast, the negative slope of the size spectrum starts just above 40 cm. Sampling in Subdivision 27 in the central Baltic Proper demonstrates a popula‐ tion with considerably higher mean length and with single individuals reaching al‐ most 90 cm in length (Figure SE.18).
572 EIFAC/ICES WGEEL Report 2008
9 SD 20 SD 21 SD 23 8 SD 25 SD 27 7 6 5 % 4 3 2 1 0 26 29 32 35 38 41 44 47 50 53 56 59 62 65 68 71 74 77 80+
cm-group
Figure SE.18 Length composition of yellow eel from commercial fykenet catches in samples col‐ lected in 2002–2007 in RBD SE Västerhavet (ICES SD 20–21) and RBD SE Baltic (ICES SD 23,25 and 27). Samples from subdivisions 25 and 27 are based on an unsorted mixture of landings and discard.
There is a gradient in mean length of silver eel from 77 cm SD 27 in central Baltic to 65 cm in SD 23, Öresund. Since May 2007 the minimum legal landing size is 65 cm in the Baltic. The length distributions in SD 24–25 in the southern Baltic indicate a po‐ tential for a considerable reduction of the fishing mortality in the poundnet fishery in this area with the new size limit.
9,0 8,0 SD 23 SD 24 SD 25 SD 27 7,0 6,0 5,0 % 4,0 3,0 2,0 1,0 0,0 38 40 42 46 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 cm-group
Figure SE.19 Length composition of silver eel from commercial poundnet catches for samples collected in 2005–2007 in RBD SE Baltic (ICES SD 23, 24, 25 and 27).
In the three western subdivisions, Öresund, Kattegat and Skagerrak, the average age of the yellow eel in commercial landings varied between 8 and 10 years. The samples from SD 25 represent the first proper habitat for yellow eel recruits on their path of migration from the west coast into the Baltic eSea. Th relatively low mean age in un‐ sorted fykenet landings in SD 25 indicate that migrants on transit might make up a considerable proportion of the catches. Although the yellow eels from SD 27 in the Central Baltic were considerably larger, they were only 1–2 years older compared to the western sampling sites. Silver eel ages varied from 14 years on average in SD 27 to 10–12 years in SD 23–25.
EIFAC/ICES WGEEL Report 2008 573
Table SE.l Mean age of yellow eel in the Swedish coastal fykenet fishery.
ICES SD Year of catch 2002 2003 2004 2005 2006 Total 20 9,0 8,9 9,6 8,7 8,9 21 8,7 8,2 8,7 7,9 9,2 8,4 23 8,6 9,6 9,4 8,9 9,1 25 7,2 6,8 7,0 27 9,8 10,9 10,1
In SD 20, 21 and 23 (West Coast) eels were recruited to the fishery at the age of 4 to 5 years and the oldest individuals recorded had reached the age of 18. On the southern Baltic coast the age span in unsorted landings was 3–12 years. The age distribution in SD 27 was similar to those from the west coast, although shifted one year to the right in Figure SE.20.
n 200 180 SD25 SD20 160 SD21 SD23 140 SD27 120 100 80 60 40 20 0 3 4 5 6 7 8 9 101112131415161820 age
Figure SE.20 Age distribution of yellow eel from commercial fykenet catches for samples col‐ lected in 2005–2006 in RBD SE Västerhavet (ICES SD 20–21) and RBD SE Baltic (ICES SD 25 and 27).
The growth pattern is close to linear for both length and weight in all areas (Figure SE.21). Bias is probably introduced for younger ages as a consequence of gear selec‐ tivity and in higher ages as a consequence of silvering. Yellow eel from SD 27 in cen‐ tral Baltic were considerably longer and heavier than in other areas, a 10‐year‐old female being 57 cm and 314 g in the former area compared to 49,5 cm and 192 g on the Skagerrak coast (SD 20). Comparing the most abundant ages, somatic condition is higher in the Baltic samples and increases with increasing age. The possibly transiting eels in SD 25 thus were in better condition than eels from the west coast, but had oth‐ erwise grown at approximately the same speed. Condition increasing with increasing age is seen in all areas but SD 20.
574 EIFAC/ICES WGEEL Report 2008
mm 900
800
700
600
500 SD20 SD21 SD23 SD25 400 SD27
300 3 4 5 6 7 8 9 101112131415161820 age
g 900 800 SD20 SD21 SD23 SD25 700 SD27 600 500 400 300 200 100 0 3 4 5 6 7 8 9 101112131415161820 age
condition factor 2,2 SD20 SD21 2 SD23 SD25 SD27 1,8
1,6
1,4
1,2
1 3 4 5 6 7 8 9 101112131415161820 age
Figure SE.21. Length, weight and condition factor at age of yellow eel from commercial fykenet catches in samples collected in 2005–2006 in RBD SE Västerhavet (ICES SD 20–21) and RBD SE Baltic (ICES SD 25 and 27).
EIFAC/ICES WGEEL Report 2008 575
1.2
1
0.8
0.6 kg
0.4
0.2
0
9 2 5 8 1 4 7 6 9 2 5 8 1 4 5 6 7 8 9 0 97 19 196 19 196 19 197 1 1980 1983 198 19 199 19 199 20 200
Hanöbukten Dragskär Marsö
Figure SE.22 Changes over time in mean total weight of silver eels from SD 25 (Hanöbukten) and SD 27 (Dragskär+Marsö).
Mean weight of silver eels in commercial poundnet catches have increased over time (Figure SE.22) from 0.6 kg in the 1960´s to 1 kg in recent years. The trend is the same in both SD 25 and SD 27 although the mean weight of silver eels is generally lower in SD 25. There are some uncertainties in the data before 1970 such that some yellow eel could be included in the statistics.
SE.H.2 Freshwater
In addition to the programme mentioned under Section SE.G.3 no data on catch com‐ position is collected in fresh waters.
SE.I Other biological sampling
SE.I.3 Parasites
The swimbladder parasite (Anguillicola) does occur in eels from most sites. All eels dissected at the Swedish Board of Fisheries are analysed macroscopically for the prevalence (at both Institutes involved) and intensity (at the Institute of Freshwater Research only) of Anguillicola in their swimbladders. The prevalence in coastal waters in 2002–2005 was close to 10% in the marine habitats of RBD 5 and about 60% in the central parts of RBD 4. The straight between Sweden and Denmark (Öresund, SD 23) took an intermediate position.
SE.H.2 Freshwater
In addition to the programme mentioned under Section SE.G.3 no data on catch com‐ position is yet collected in fresh waters. However, the intention is to monitor both catch and the yellow eel stock within the coming DCR‐programme.
576 EIFAC/ICES WGEEL Report 2008
Prevalence of Anguillicola crassus is a mandatory variable in all coastal sampling of eel in Sweden, including the DCR sampling. The rate of infestation in the pooled data from 2002–2006 was less than 15% in the most marine areas, 47% in Öresund and close to 60 in the Baltic sites.
% 70
60
50
40
30
20
10
0 SD20 SD21 SD23 SD25 SD27
Figure SE.23 Prevalence of the swimbladder parasite (Anguillicola crassus) in yellow eel from commercial fykenet catches for samples collected in 2005–2006 in RBD SE Västerhavet (ICES SD 20–21) and RBD SE Baltic (ICES SD 25 and 27).
Table SE.m Prevalence of Anguillicola crassus in yellow eel from Swedish coastal waters in 2002– 2005. ICES Subdivisions 20–21 represent RBD 5, other Subdivisions represent RBD 4.
ICES Subdivision 20 21 23 25 27 Not infested 723 611 442 475 493 Infested 80 93 361 753 794 Grand Total 803 704 803 1228 1287 Prevalence 0.10 0.13 0.45 0.61 0.62
Between 2000 and 2008 the Institute of Freshwater Research analysed 3608 eels from 41 different fresh‐water sites. Infested eels were found in all sites and the prevalence varied from 37% to 91%.
SE.I.4 Contaminants
The National Food Administration in Sweden has analysed both yellow and silver eels sampled in 2000 and 2001 from nine different sites in Sweden with respect to 17 dioxins and furans and 10 dioxin‐like PCB congeners (www.slv.se). Pooled samples revealed that eels had less than 1 pg TEQ/g fresh weight of sum TCDD/F in muscle (TEQ = Toxic Equivalents, TCDD = C12H4O2Cl4). To this came about 3.8 pg PCB‐ TEQ/g fresh weight. Silver eels had higher levels than yellow ones. Compared to the other fish species analysed, eels have a higher ratio of PCB to dioxins. Due to the high costs for this type of analyses only few eels will be sampled regularly in future.
Recently yellow eels from the Sound (between Sweden and Denmark) outside a heav‐ ily loaded industrial area in Helsingborg were analysed for dioxins and dioxin‐like PCBs. Pooled samples from 2005 contained 5.7 WHO‐PCDD/F‐TEQ pg/g and 11 WHO‐PCB‐TEQ pg/g, both based on fresh weights. In 2006 another five pooled sam‐ ples from the same area were analysed. The dioxins varied between 0.9 and 4.7 with
EIFAC/ICES WGEEL Report 2008 577
an average of 2,2 WHO‐PCDD/F‐TEQ pg/g. The PCBs varied between 3.9 and 12.7 with an average of 6,6 WHO‐PCDD/F‐PCB‐TEQ. At some sites the level of dioxins in eel muscle exceeded by that the 4 p/g level of dioxins or the 12 pg/g level of summed up dioxins and dioxin‐like PCBs, set as maximum allowed levels in eel by the Com‐ mission of the European Communities. In 2007 further samples were analysed from this area. Both yellow and silver eels were analysed in seven pooled samples. The dioxin levels varied between 0,6 and 2,7 pg/g and the summed up dioxins and dioxin‐ like PCBs between 2.3 and 8.3 pg/g, i.e. all below the maximum allowed levels. How‐ ever, the sample sites were not exactly the same as in 2005 and 2006 (Source: SLV (The National Food Administration)).
Recent analyses of mercury (Hg) in eels from a number of lakes did demonstrate very low levels.
SE.I.5 Predators
Cormorants
Cormorants are believed to predate substantially on eels. As about 2900 young eels stocked in Lake Ymsen 1998–2000 were equipped with PIT‐tags in spring 2004 we took the opportunity to scan the ground below the only cormorant colony in that lake for tags. In total 30 PIT‐tags were found corresponding to a minimum loss by cormo‐ rant predation of 1%.
An extensive study of the stomach content of cormorants at three sites along the Kat‐ tegat‐Skagerrak coast revealed that eels were taken by about 5% of the cormorants. That was equivalent to about 1% of their diet. Despite the low percentage, it corre‐ sponds to a total annual predation of 310 000 yellow eels, i.e. one fourth of the com‐ mercial catch on this coast (Lunneryd and Alexandersson, 2005).
Pellets from cormorants were analysed from a colony outside River Dalälven. No re‐ mains from eel were discovered. However, it is known that this approach is not that suitable for eel as their otoliths are easily eroded (Bostrom et al., in press).
Seals
Along the Swedish West Coast there is substantial damage on eel fykenets done by harbour seal (Phoca vitulina) Königson et al., 2006. The cost of the damage estimates to several per cent (up to 18%) of the catch (Königson et al., 2003). There are circum‐ stances that indicate that the raiding seals are a minor part of the population. It is demonstrated that those seals have strong preference for eel compared with cod or flatfish in the fykenets (Königson et al., 2006). Old diet studies indicate that a “nor‐ mal” seal seldom eat eel (Härkönen and Heide-Jørgensen, 1991) but obvious is that the specialised seals that damage the fykenets cause an additional mortality on the eel population of several per cent of the catches.
There is only one minor diet study of grey seals (Halichoerus grypus) in the Baltic proper. The material consists of fish remains from 54 stomachs and intestines which reflect maximum one day’s food. Remains were found from two eels (Lundström et al., in press). It is from those figures impossible to calculate an accurate figure of how important eels are fore the grey seals.
578 EIFAC/ICES WGEEL Report 2008
SE.J Other sampling
SE.J.2 Obstacles to eel migration
During 2005 and 2006 an inventory of obstacles for eels migrating both up‐ and downstream was performed. Not only are the obstacles as such studied but also the occurrence of fish passes, by‐passes, deflecting screens, etc. and their suitability for eels were noted. The purpose is to achieve a database to be used as background when installing new or improving existing eel passes and deflecting devices. Parts of the Swedish eel management plan are based on data in this database. Water Courts deci‐ sions might also be reconsidered with this database as argument.
SE.K Stock assessment
So far the collected data has not by routine been used for stock assessment.
Published mortality estimates from Subdivision 20 and 21 (Svedäng, 1999) (approxi‐ mating RBD 5, Västerhavets vattendistrikt (“the North Sea”)) have been used in a simple length based mortality rate model to assess the effect of present yellow eel exploitation on spawner escapement in relation to present and estimated past unex‐ ploited levels of spawner escapement (Åström and Wickström, 2004). The relation between the present and past population levels has been estimated using the longer dataseries on ascending elvers and young eels, indicating that the present population probably is less than 10% of the one in the mid‐1900s.
An attempt has also been made to use the length sampling from the yellow eel fishery in fives areas in ICES Subdivision 25 and 27 (part of RBD 4, Södra Östersjöns vat‐ tendistrikt (“the Southern Baltic Sea” or SBAL)) in a catch‐at‐length analysis to esti‐ mate natural and yellow eel fishery induced instantaneous mortality rates, in terms of mortality rate per unit length increment. The result from analyses of a large number of mark recapture studies on silver eels has been used as a rough estimate of the sil‐ ver eel fishery mortality rate. Data on average length of female silver eels in the sub‐ divisions were also needed for the analyses. Males have been disregarded because of their very low prevalence in Swedish waters. The simple length based mortality rate model has then been used to assess the effect of present yellow and silver eel exploi‐ tation on spawner escapement in subdivision 25 and 27 in relation to present and es‐ timated past unexploited levels of spawner escapement (Åström, 2004).
The above analyses indicate that the yellow eel exploitation allows at most 15% of the present possible escapement to the silver eel stage. This applies both to Subsections 20 and 21 (~ RBD 5) as well as to areas where yellow eels are fished in Subsections 25 and 27 (part of RBD 4), and indicates a severe overexploitation. In the latter area (the coast of the Baltic Sea) the yellow eel exploitation is however only occurring scattered and locally (in 2006 approximately 187 600 kg was caught), so the over all effect of the yellow eel fishery in subsection 25 and 27 is not as severe as on the Swedish west coast. The silver eel fishery in Subsections 25 and 27 then reduces the spawner es‐ capement by about 36%, so that only about 11% of the currently possible spawner escapement remains of eels from areas where yellow and silver eel fishery occur. In perspective of past possible spawner escapement this would only amount to about 1% of the spawner escapement possible in the mid‐1900s.
Using additional data on the amounts of yellow and silver eels caught in the different subdivisions have allowed for analyses of the possible effects of fishing restrictions and re‐stocking of elvers on spawner escapement using the same conceptual model (Åström, 2005).
EIFAC/ICES WGEEL Report 2008 579
SE.N Overview
To some extent Sweden has a good data situation, particularly regarding coastal yel‐ low eels. At the same time much remains to be filled in order to be able to establish a sustainable management in accordance with the EU regulation regarding eel man‐ agement. The Department of Research and Development of the Swedish Board of Fisheries has recently changed its system for planning and prioritizing allowing for coherent planning, collection of data and analyses. The planning for the sampling of the fishery, monitoring of population status and evaluation of management efforts remain to be done during autumn of 2008.
SE.O Literature references
CEN 2002 Water analysis‐Sampling fish with electricity. CEN/TC 230/WG 2/TG 4 N. Final draft for formal vote.
Degerman, E., Fogelgren, J.E., Tengelin, B. and E. Thörnelöf 1985 Förekomst och täthet av havsöring, lax och ål i försurade mindre vattendrag på svenska västkusten [Occurrence of brown trout, Atlantic salmon and eel in small acidified watercourses on the West Coast of Sweden]. Information från Sötvattenslaboratoriet, Drottningholm. 65 p.
Fiskeriverket 2005 Fiske 2005, en undersökning om svenskarnas fritidsfiske, Fiskeriverket i samarbete med SCB. 89 p. [Fishing 2005, a survey of the Swede’s recreational fishing].
Fiskeriverket and Laxforskningsinstitutet 1999 RASKA‐Resursövervakning av sötvattensfisk. [English summary: The status of fish populations in inland waters and coastal rivers in Sweden]. Inform.Inst.Freshw.Res., Drottningholm (8). 37 p.
Hagström O. and H. Wickström 1990 Immigration of Young Eels to the Skagerrak‐Kattegat Area 1900 to 1989. Int. Revue. Ges. Hydrobiol. 75 86): 707–716.
Härkönen, T., and Heide‐Jørgensen, M. P. 1991. The harbour seal Phoca vitulina as a predator in the Skagerrak. Ophelia, 34: 191–207.
Königson, S., Lunneryd, S.G., and Lundström, K. (2003) Sälskador i ålfisket på svenska västkusten. En studie av konflikten och dess eventuella lösningar. (The seal‐fisheries con‐ flict on the west coast of Sweden. An investigation of the problem and its possible solu‐ tions. (In Swedish with an English summary)). Finfo. Fiskeriverket informerar. 9, 1–24. http://www.fiskeriverket.se/service/publikationer/fiskeriverketinformerar/finfo2003/finfo20039.4.63 071b7e10f4d1e2bd380006462.html.
Königson, S., Lundström, K., Hemmingsson, M., Lunneryd, S.G., and Westerberg, H. 2006. Feeding Preferences of Harbour Seals (Phoca vitulina) Specialised in Raiding Fishing Gear. Aquatic Mammals, 32(2): 152–156.
Lundberg, D. 2008. Stort illegalt ålfiske i sjöar och åar. Yrkesfiskaren 5: 7.
Lunneryd, S.‐G. and Alexandersson, K. 2005. Födoanalyser av storskarv, Phalacrocorax carbo i Kattegatt–Skagerrak. Finfo 2005:11. 22p.
SCB (Statistics Sweden) 2005. Vattenbruk 2004 [Aquaculture in Sweden 2004]. Sveriges Officiella Statistik, Statistiska Meddelanden JO60SM0501, 17p.
Svedäng H. 1999 Vital population statistics of the exploited eel stock on the Swedish west coast. Fisheries Research 40: 251–265.
Westerberg, H. 1998a. Oceanographic aspects of the recruitment of eels to the Baltic Sea. Bull. Fr. Pêche Piscic. 349: 177–185.
Westerberg, H. 1998b. The migration of glass‐eel and elvers in the Skagerrak and the Kattegatt. ICES CM 1998/N:11, 14 p.
580 EIFAC/ICES WGEEL Report 2008
Westerberg H., J. Haamer and I. Lagenfelt 1993 A new method for sampling elvers in the coastal zone. ICES C.M. 1993/M:5, 10p.
Wickström, H. 2002. Monitoring of eel recruitment in Sweden. Volume 2A: Country reports, Northern part: 69–86. In: Monitoring of glass eel recruitment. Dekker, W. (Ed). Nether‐ lands Institute of Fisheries Research, IJmuiden, The Netherlands, Report C007/02‐WD, 256 pp.
www.slv.se 2004 Delrapport 3‐dioxinanalyser av fet fisk från Sverige 2001–2002 [Report 3– analyses of dioxins in fat fish from Sweden 2001–2002] http://www.slv.se/templates/SLV_Page.aspx?id=9624.
Åström, M. 2004. ʺAnalyser rörande ålen och ålfisket i svenska kustvattenʺ. Intern rapport inom Fiskeriverket. [Analyses concerning the eel and the eel fishery in coastal waters of Sweden. Internal report of the Swedish Board of Fisheries].
Åström, M. 2005. Spawner escapement from yellow and silver eel fishery. Appendix 3.3. in ICES CM 2005/I:01, Report on the ICES/EIFAC Working Group on Eels, Galway (WGEEL), 22–26 November 2004, Galway, Ireland.
Åström, M. and Wickström, H. 2004. Some management options for the yellow eel fishery on the Swedish west coast. Internal report of the Swedish Board of Fisheries.
EIFAC/ICES WGEEL Report 2008 581
Appendix
Table SE.n Commercial landings of eel in Sweden (Kattegat‐Skagerrak corresponds to RBD 5 and the data come from the contract notes). (cf Figure SE.4).
YEAR SOUTH C. (BALTIC SEA) EAST C. (BALTIC SEA) KATTEGAT-SKAGERRAK FRESHWATER TOTAL SWEDEN
1925 624 936 155 1715 1926 520 1011 176 1707 1927 642 1216 152 2010 1928 373 509 157 1039 1929 582 644 167 1393 1930 716 596 216 1528 1931 782 497 252 1531 1932 769 701 253 1723 1933 645 704 196 1545 1934 798 830 215 1843 1935 829 880 240 1949 1936 608 818 226 1652 1937 548 931 244 1723 1938 666 969 235 1870 1939 535 988 248 1771 1940 553 974 98 1625 1941 633 926 69 1628 1942 426 592 110 1128 1943 820 648 77 1545 1944 879 1042 79 2000 1945 778 790 96 1664 1946 658 738 116 1512 1947 980 761 169 1910 1948 979 689 194 1862 1949 999 671 229 1899 1950 1109 911 168 2188 1951 962 755 212 1929 1952 791 627 180 1598 1953 1146 879 353 2378 1954 1186 780 140 2106 1955 1599 780 272 2651 1956 714 707 112 1533 1957 1158 856 211 2225 1958 938 642 171 1751 1959 1658 977 154 2789 1960 778 703 165 1646 1961 896 870 300 2066 1962 980 713 215 1908 1963 997 802 272 2071
582 EIFAC/ICES WGEEL Report 2008
YEAR SOUTH C. (BALTIC SEA) EAST C. (BALTIC SEA) KATTEGAT-SKAGERRAK FRESHWATER TOTAL SWEDEN 1964 1303 749 236 2288 1965 749 768 285 1802 1966 748 893 328 1969 1967 646 703 268 1617 1968 713 794 301 1808 1969 622 733 320 1675 1970 476 515 318 1309 1971 545 587 259 1391 1972 425 582 197 1204 1973 419 553 240 1212 1974 322 470 242 1034 1975 494 629 276 1399 1976 283 363 289 935 1977 346 340 303 989 1978 376 385 315 1076 1979 267 404 285 956 1980 371 438 303 1112 1981 243 153 491 887 1982 342 250 569 1161 1983 267 171 735 1173 1984 559 136 378 1073 1985 647 213 280 1140 1986 479 138 234 92 943 1987 439 119 250 89 897 1988 532 190 304 136 1162 1989 447 132 264 109 952 1990 452 119 242 129 942 1991 486 181 285 132 1084 1992 534 162 352 132 1180 1993 550 93 438 129 1210 1994 654 98 630 171 1553 1995 444 79 555 127 1205 1996 564 67 406 97 1134 1997 546 181 513 142 1382 1998 318 50 165 112 645 1999 339 69 186 140 734 2000 286 39 123 113 561 2001 107 123 195 118 543 2002 126 183 222 102 633 2003 115 145 209 96 565 2004 84 134 227 106 551 2005 119 187 211 111 628 2006 125 195 227 123 670 2007 126 178 153 111 568
EIFAC/ICES WGEEL Report 2008 583
Table SE.o Total commercial landings (tonnes) in coastal fishery by RBD. (cf Figure SE.2).
YEAR BBAY BSEA NBAL SBAL WEST BBAY+BSEA
1999 0 3.0446 44.2675 265.5355 247.427 3.0446 2000 0.028 2.7171 31.5765 221.2225 161.4925 2.7451 2001 0 3.1427 28.1985 263.8105 227.71 3.1427 2002 0.015 3.05 29.337 239.6801 216.791 3.065 2003 0.003 4.2107 25.0735 244.5234 193.616 4.2137 2004 0.0015 4.2873 22.3375 224.2218 219.357 4.2888 2005 0 3.5522 38.0145 303.818 215.2515 3.5522 2006 0.109 3.5769 30.8573 329.8463 240.3054 3.6859 2007 0.0645 1.207 43.4387 371.4447 172.287 1.2715
584 EIFAC/ICES WGEEL Report 2008
Table SE.p Total effort (number of gears* number of fishing nights) in pound nest in a Subarea in SD 27. (cf Figure SE.3).
YEAR GEARS*NIGHTS
1962 3334 1963 4710 1964 5186 1965 4004 1966 4834 1967 5915 1968 5749 1969 6001 1970 5659 1971 5232 1972 4697 1973 4958 1974 4689 1975 4756 1976 3596 1977 3563 1978 3438 1979 2566 1980 3404 1981 3260 1982 2771 1983 3269 1984 3435 1985 2762 1986 3158 1987 3559 1988 2772 1989 2587 1990 2290 1991 2517 1992 2538 1993 2397 1994 2362 1995 2157 1996 2206 1997 1894 1998 1964 1999 1493 2000 1558 2001 1532 2002 1062 2003 973 2004 1535 2005 1311 2006 1464
EIFAC/ICES WGEEL Report 2008 585
Table SE.q Mean length‐at‐age with standard deviation in Swedish samples of yellow eel from commercial fykenet catches in 2002–2005.
Mean length in mm SD 20 SD 21 SD 23 SD 25 SD 27 Stenungsund Kullen Öresund Valjeviken Simpevarp Kvädöfjärden 2002 2003 2004 2005 2002 2003 2004 2005 2003 2004 2005 2003 2004 2004 2004 3 336 341 4 403 407 357 385 5 396 342 376 403 374 423 428 399 409 369 381 499 6 378 375 375 396 429 427 449 447 464 444 412 413 435 526 413 7 419 396 441 436 446 447 463 465 432 418 471 414 466 535 485 8 416 418 471 466 451 462 484 499 460 448 491 444 461 549 525 9 459 428 478 513 454 500 501 508 485 457 504 496 484 564 545 10 436 496 484 572 492 525 511 576 508 497 524 477 492 591 553 11 493 502 497 565 541 549 546 590 534 512 527 529 654 559 12 484 543 668 523 524 655 654 521 566 524 628 639 599 13 523 561 573 711 576 561 635 579 562 614 675 609 14 475 547 496 604 565 614 582 568 589 682 645 15 726 512 585 606 568 693 624 16 617 678 17 18 778 20 778
Standard Devation SD 20 SD 21 SD 23 SD 25 SD 27 Stenungsund Kullen Öresund Valjeviken Simpevarp Kvädöfjärden Age 2002 2003 2004 2005 2002 2003 2004 2005 2003 2004 2005 2003 2004 2004 2004 3 72 37 4 - 19 37 42 5 - 20 30 91 18 14 17 61 80 28 33 62 645454930616418354968225537 45 41 738534453655151344440613455 64 56 849664460595644603350413354 72 50 970544656397843694850534837 62 59 10 56 67 49 51 74 70 71 68 64 56 50 49 43 62 67 11 62 48 32 55 59 70 49 57 41 63 64 111 87 61 12 26 34 67 72 63 102 49 38 77 92 - 100 60 13 95 80 41 - 70 - - 15 98 64 58 58 14 - 2 56 - 101 - 15 6 16 68 82 15 -- --- 112 38 16 - - 17 18 - 20 -
586 EIFAC/ICES WGEEL Report 2008
Table SE.r Mean weight (kg) of silver eels in SD 25 (Hanöbukten) and SD 27 (Dragskär+Marsö). (cf Figure SE.22).
HANÖBUKTEN DRAGSKÄR MARSÖ
1959 0.4257096 1960 0.3812911 1961 0.3944881 1962 0.3841353 0.646057 0.581714 1963 0.3933078 0.66662 0.596092 1964 0.381971 0.656284 0.6516 1965 0.4028978 0.668809 0.617855 1966 0.3956977 0.66507 0.818465 1967 0.3982816 0.666319 0.64349 1968 0.4206718 0.665281 0.643382 1969 0.45799 0.669758 0.67301 1970 0.4487651 0.797074 0.693331 1971 0.4985409 0.888208 0.704245 1972 0.4767305 0.795598 0.737115 1973 0.4437471 0.809352 0.785968 1974 0.5302373 0.836614 0.803108 1975 0.5363621 0.857662 0.842197 1976 0.5226509 0.86879 0.80943 1977 0.5831722 0.9 0.818641 1978 0.910007 0.840489 1979 0.949199 0.869809 1980 0.968704 0.868633 1981 0.6134633 0.9166 0.84257 1982 0.5912612 0.934878 0.866136 1983 0.6886279 0.943427 0.890408 1984 0.5686305 0.952998 0.899468 1985 0.601751 0.95387 0.894093 1986 0.6386582 0.951868 0.8808 1987 0.6384719 0.947937 0.909734 1988 0.6478994 0.946292 0.929888 1989 0.6082842 0.919714 0.928396 1990 0.6707184 0.960589 0.963711 1991 0.694523 0.941953 0.980984 1992 0.678391 1.010102 0.985237 1993 0.7145674 1.023795 1.029801 1994 0.7589975 0.944953 1.038153 1995 0.7438935 0.942792 1.039462 1996 0.7227103 0.949406 1.002065 1997 0.7161557 0.956877 1.011255 1998 0.7193059 0.958333 0.995137 1999 0.7029799 0.980412 2000 0.7044675 1.034976
EIFAC/ICES WGEEL Report 2008 587
HANÖBUKTEN DRAGSKÄR MARSÖ 2001 1.0817297 1.059891 2002 0.6769622 0.98806 2003 0.9994292 0.904513 2004 0.7962425 1.007576 2005 0.801855 2006 0.7786137